Unit pixel of image sensor and light-receiving element thereof

Abstract

Provided are a light-receiving element which has more capability of detecting wavelengths than that of existing silicon light-receiving elements and a unit pixel of an image sensor by using it. The light-receiving element includes: a light-receiving unit which is floated or connected to external voltage and absorbs light; an oxide film which is formed to come in contact with a side of the light-receiving unit; a source and a drain which stand off the light-receiving unit with the oxide film in between and face each other; a channel which is formed between the source and the drain and forms an electric current between the source and the drain; and a wavelength expanding layer which is formed in at least one among the light-receiving unit, the oxide film and the channel and forms a plurality of local energy levels by using strained silicon.

What is claimed is: 
     
         1 . A unit pixel of an image sensor, comprising:
 a light-receiving element which generates an electric current by using change of electric charge quantity caused by incident light;   a selection element which outputs the electric current generated in the light-receiving element to a unit pixel output; and   a reset element which removes residual electric charges from the light-receiving element,   wherein the light-receiving element comprises:   a light-receiving unit which absorbs light;   an oxide film which contacts a side of the light-receiving unit;   a source and a drain which stand off the light-receiving unit with the oxide film in between the source and the drain, and the source and the drain facing each other;   a channel region formed between the source and the drain such that an electric current can be formed between the source and the drain; and   at least one wavelength expanding layer, formed in at least one among the light-receiving unit, the oxide film, and the channel region, and the at least one wavelength expanding layer split energy levels to form a plurality of local energy levels,   wherein the wavelength expanding layer comprises strained silicon and splits the energy levels of the conduction band and the valence band according to the direction of crystal lattice planes of silicon.   
     
     
         2 . The unit pixel of an image sensor of  claim 1 , wherein:
 the light-receiving unit is structured to be floated;   the light-receiving unit is electrically insulated from the source and the drain by the oxide film;   when a local energy level condition of at least one of the plurality of local energy levels is met, electrons excited by received light flow into the light-receiving unit under a tunneling effect; and   the electric current along the channel region is controlled according to change of electric charge quantity in the light-receiving unit from the flow of the electrons.   
     
     
         3 . The unit pixel of an image sensor of  claim 1 , wherein the light-receiving element is formed on a silicon substrate in which first-type impurities are doped. 
     
     
         4 . The unit pixel of an image sensor of  claim 3 , wherein the source and the drain are formed on a well in which second-type impurities are doped and the well is floated. 
     
     
         5 . The unit pixel of an image sensor of  claim 1 , wherein the light-receiving element comprises a plurality of the wavelength expanding layers, which are different from each other in terms of at least one of their formation location and manufacturing process. 
     
     
         6 . The unit pixel of an image sensor of  claim 1 , wherein the wavelength expanding layer is formed by applying stress onto an interface of a silicon layer and the oxide film. 
     
     
         7 . The unit pixel of an image sensor of  claim 1 , wherein the wavelength expanding layer is formed by bonding heteroatoms to or injecting ions into a silicon crystal which forms the light-receiving unit or the channel region. 
     
     
         8 . The unit pixel of an image sensor of  claim 1 , wherein the size of the light-receiving unit corresponds to a de Broglie wavelength of an electron. 
     
     
         9 . The unit pixel of an image sensor of  claim 1 , wherein the selection element comprises:
 a source and a drain which are connected to the light-receiving element and the unit pixel output, respectively; and   a gate to which selection signals are applied from outside,   and performs switching operation on the basis of the applied selection signals.   
     
     
         10 . The unit pixel of an image sensor of  claim 1 , wherein the reset element removes residual charges from a diffusion well where the light-receiving element is formed. 
     
     
         11 . The unit pixel of an image sensor of  claim 10 , wherein the diffusion well remains floated while the light-receiving element operates. 
     
     
         12 . The unit pixel of an image sensor of  claim 1 , further comprising a terminal which applies gate voltage to the light-receiving unit so as to control the threshold voltage and output signals by controlling a gap between a Fermi level and an intrinsic level of the channel region. 
     
     
         13 . The unit pixel of an image sensor of  claim 1 , wherein the tunneling occurs in an oxide film zone between any one of the channel region and the drain, and the light-receiving unit. 
     
     
         14 . The unit pixel of an image sensor of  claim 1 , wherein the light-receiving element can detect light with wavelengths exceeding 1,100 nm. 
     
     
         15 . The unit pixel of an image sensor of  claim 1 , wherein the light-receiving element has peak detection characteristics at wavelengths of 900 nm or more.